[from the nuclear power industry] An emergency-power-off switch (see
Big Red Switch), esp. one positioned to be easily
hit by evacuating personnel. In general, this is not
something you frob lightly; these often initiate
expensive events (such as Halon dumps) and are installed in a
dinosaur pen for use in case of electrical fire or
in case some luckless field servoid should put 120
volts across himself while Easter egging. (See also
molly-guard, TMRC.)

“Scram” was in origin a backronym for “Safety Cut
Rope Axe Man” coined by Enrico Fermi himself. The story goes that
in the earliest nuclear power experiments the engineers recognized the
possibility that the reactor wouldn't behave exactly as predicted by their
mathematical models. Accordingly, they made sure that they had mechanisms
in place that would rapidly drop the control rods back into the reactor.
One mechanism took the form of ‘scram technicians’. These
individuals stood next to the ropes or cables that raised and lowered the
control rods. Equipped with axes or cable-cutters, these technicians stood
ready for the (literal) ‘scram’ command. If necessary, they
would cut the cables, and gravity would expeditiously return the control
rods to the reactor, thereby averting yet another kind of core
dump.

Modern reactor control rods are held in place with claw-like devices,
held closed by current. SCRAM switches are circuit breakers that
immediately open the circuit to the rod arms, resulting in the rapid
insertion and subsequent bottoming of the control rods.